Sheets and strips of plain carbon steel compositions have been used in forming body structural members and body panels for automotive vehicles for many years. Such steel workpieces can be stamped or otherwise formed into the various, often complicated body member shapes and display strengths required of such manufactures. But with the increasing need to reduce vehicle weight for improved fuel economy it has been necessary to reduce thicknesses of the steel sheets and strips and to increase the formability of such workpieces, while seeking to obtain even higher strengths in the formed vehicle body components and other structures.
In accordance with an American Iron and Steel Institute description, “Steel is considered to be carbon steel when no minimum content is specified for chromium, cobalt, molybdenum, nickel, niobium, titanium, tungsten, vanadium, or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 percent; or when the maximum content for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.” The carbon content is not specified in this definition. Low alloy steels typically contain small amounts of one or more of manganese, nickel, chromium, molybdenum, vanadium, and silicon. For example, a representative, low carbon, low alloy steel may be composed of, by weight, 0.25% max carbon, 0.4% to 0.7% manganese, 0.1% to 0.5% silicon, and the balance iron except for trace amounts of other elements introduced though re-cycling and other processing of starting material.
In preparation for making automotive vehicle body components, such plain and low carbon steel compositions are shaped from cast ingots into rolls of sheets or strips by a combination of hot rolling and cold rolling operations. Depending on their thermal and mechanical processing history, such hot and cold rolled steels may have a variety of microconstituents at ambient temperatures. Such microconstituents may comprise ferrite (α-iron)—a body-centered cubic crystal structure of iron atoms; iron carbide or “cementite;” retained austenite (γ-iron)—a face-centered cubic crystal structure of iron atoms with dissolved carbon; and martensite—a metastable body-centered phase of iron supersaturated with carbon, produced through a diffusionless phase change by quenching austenite. A typical microstructure produced by cooling the high temperature austenite phase at moderate cooling rates would consist of proeutectoid ferrite (ferrite which separates from hypoeutectoid austenite above the eutectoid temperature) and pearlite or bainite, or more generally, a combination of these constituents. Pearlite is formed by cooperative growth of alternating ferrite and cementite lamellae from austenite of eutectoid composition (iron with 0.8% by weight carbon) at relatively small undercooling. Bainite is formed from austenite at higher undercooling and consists of ferrite plates in combination with fine carbides precipitated either between or inside the plates. At sufficiently high cooling rates the transformation of the austenite to ferrite, pearlite and bainite can be precluded by its transformation to the metastable martensite phase by a diffusionless shear transformation. Depending on the steel composition, cooling rate and quench temperature, a portion of the austenite phase can be retained in the microstructure at ambient temperatures. Among the parameters that encourage the stability of retained austenite are high carbon content and a fine grain size.
In order to obtain a microstructure suitable for a subsequent sheet forming operation, the cold rolled, ferritic steel workpieces are typically heated above their respective A3 temperatures (e.g., close to 900° C., depending on the composition of the steel alloy) to obtain a uniformly austenitic crystal structure and then quenched below their Ms Temperature (e.g., about 400° C., again depending on the steel composition) to convert a portion of the austenite phase to martensite. The resulting proportions of newly formed martensite and retained austenite affect the formability and strength of the steel workpiece. Such a heat treatment practice may be performed by the steel supplier or by the manufacturer that is going to deform the steel sheet or strip material into a stamped or otherwise shaped product. The manufacturer of the vehicle body components obtains the sheet or strip material, and cuts suitable sections from it for the forming of the parts. The parts may be shaped at an ambient temperature in a stamping plant or formed in a heated press or other metal-forming machine.
The strip or sheet workpieces have become progressively thinner as higher strength steel microstructures have been produced. A goal of a steel processer into vehicle body components is to start with a low alloy steel workpiece that is highly formable at a desired forming temperature (typically an ambient temperature) and then to produce a formed steel part that is very strong and of light weight. But these two goals of initial low strength and high formability and final complex shape and high strength have been difficult to attain. Sheet steels designed specifically to meet the more recent demands for better combinations of high strength and ductility have been categorized as Advanced High Strength Steels.
One approach to achieving Advanced High Strength Steels with the necessary combination of both increased strength and increased ductility relies on the ability to retain the high temperature austenite phase in the steel microstructure prior to forming. Upon quenching the austenitic steel there is a tendency for some of the high temperature austenite phase to be retained as such in the quenched microstructure rather than transform to the martensite phase or other austenite decomposition products. Steels specifically alloyed and processed to contain a significant amount of retained austenite can undergo Transformation Induced Plasticity whereby strain induced transformation of the retained austenite during forming results in greater levels of both strength and ductility. Steels can be specially formulated and processed in order to maximize the amount of retained austenite in the starting steel sheet, and thus take best advantage of the Transformation Induced Plasticity, or “TRIP” effect, which improves the ductility of the steel. As the TRIP steel is formed at room temperature, the retained austenite in the severely strained regions of the part will transform to martensite. The result is that the rate of work hardening is increased in those regions of the part which inhibits local thinning or “necking” and thus increases the ductility or formability of the steel. Steels can be formulated and processed in order to retain greater amounts of austenite prior to deformation thus achieving greater combinations of strength and ductility in the formed parts. Formulation of such a steel composition may include on the order of up to 0.4% C and 1.5% Mn. In addition to increasing both strength and hardenability of the steel, both C and Mn are strong austenite-stabilizing alloying elements which reduce the martensite start temperature and encourage the retention of austenite upon quenching. A steel alloy designed for the purpose of retaining a large fraction of austenite may also contain on the order of 1% Si or Al to suppress the formation of carbides which would otherwise deplete the carbon content of the retained austenite making it less stable at room temperature.
The prior practices for retaining austenite in the low alloy steel sheet material prior to the forming of the steel workpieces have used a standard austenitization heat treatment, or, alternatively, preheating the steel in the two-phase intercritical temperature range as the initial processing step prior to quenching. There remains a need for improved methods of retaining and/or modifying austenite in low alloy content steel sheets and strips so that they can more readily be formed into complex three-dimensional shapes that display high strength and rigidity for vehicle applications and other uses.